1. Field of the Invention
The present invention relates to an apparatus and a method adopted to control the drive of an AC motor.
2. Description of the Related Art
A current control circuit that controls the current in a three-phase alternating current motor normally performs the control arithmetic operation by converting a three-phase alternating current to a direct current in order to facilitate the arithmetic operation (see Japanese Laid-Open Patent Publication No. H 08-331885). FIG. 33 shows the structure adopted in a three-phase alternating current motor control apparatus which constitutes the related art. In the control arithmetic operation performed in this motor control apparatus, a rotating orthogonal coordinate system (dq coordinate system) is used. In the rotating orthogonal coordinate system, the direction of the exciting current component in the current flowing through the three-phase alternating current motor is set along the d-axis and the direction of the torque current component in the current flowing through the three-phase alternating current motor is set along the q-axis perpendicular to the d-axis. The current control arithmetic operation is performed using a direct current value obtained by converting the three-phase alternating current value in the rotating orthogonal coordinate system to reduce the current control deviation.
Miniaturization and higher efficiency are achieved with regard to an AC motor by providing a rotor having internally embedded magnets, as shown in FIG. 34 and a stator assuming a concentrated windings structure. The rotor is able to effectively utilize the magnetic torque and the reluctance torque. A motor having this type of rotor is called an IPM (interior permanent magnet motor). The stator is capable of greatly reducing the coil end. A motor provided with the rotor and the stator having structures described above, which is called a concentrated winding IPM motor, has been attracting a great deal of interest as a motor capable of realizing miniaturization and a high degree of efficiency.
However, since the concentrated winding IPM motor described above, in which significant space harmonics manifest, poses various problems as detailed below. The space harmonics in a motor assuming a concentrated winding structure such as a concentrated winding IPM motor becomes significant because the small number of slots at each pole results in a more uneven distribution of the magnetic flux compared to that in a motor assuming a distributed winding structure. An explanation is given below as to why a uniform magnetic flux distribution cannot be achieved.
FIG. 35 shows an SPM motor assuming a surface magnet structure achieved by covering the surface of the rotor with magnets. Unlike in the SPM motor shown in FIG. 35, areas where magnets are embedded and areas where no magnet is embedded are present along the circumference of the rotor in the IPM motor with the internally embedded magnets shown in FIG. 34. Accordingly, while a uniform magnetic flux distribution is achieved in the SPM motor having a rotor, the surface of which is covered with magnets, a greater degree of change in the magnetic flux manifests in the IPM motor to result in a greater space harmonics component.
Greater space harmonics in the motor leads to a greater higher harmonics component in the current flowing to the motor, and thus, problems arise in that the extent to which the motor efficiency is improved is compromised and in that the extent of torque ripple becomes significant. In addition, since the harmonics component is superimposed on the fundamental wave component in the current, another problem manifests in that the current peak value increases. Namely, the motor control performance becomes poor due to the adverse effect of the space harmonics.
An object of the present invention is to provide a motor control apparatus and a motor control method that improve control performance in a motor with significant space harmonics.
The motor control apparatus according to the present invention comprises a fundamental current command value determining device that determines a fundamental current command value for a motor current based upon, at least, a torque command value for an AC motor, a higher harmonics current command value determining device that determines a higher harmonics current command value for the motor current and a current control device that controls the current flowing to the AC motor based upon the fundamental current command value and the higher harmonics current command value.
In the motor control method according to the present invention, a fundamental current command value for a motor current is determined based upon at least a torque command value for an AC motor, a higher harmonics current command value for the motor current is determined and the current flowing to the AC motor is controlled based upon the fundamental current command value and higher harmonics current command value.